Chemical Bonding: Ionic, Covalent and Metallic Bonding | 化学键：离子键、共价键与金属键

📚 Chemical Bonding: Ionic, Covalent and Metallic Bonding | 化学键：离子键、共价键与金属键

Understanding how atoms join together is fundamental to explaining the properties of all substances. In A-Level Combined Science, bonding types are classified into ionic, covalent, and metallic, each arising from different electron behaviour. This article breaks down bonding theories, shapes, polarity, intermolecular forces, and physical properties, helping you master this core topic.

理解原子如何结合是解释所有物质性质的基础。在 A-Level 综合科学中，键合类型分为离子键、共价键和金属键，每一种都源于不同的电子行为。本文详细解析键合理论、分子形状、极性、分子间作用力以及物理性质，助你掌握这一核心主题。

---

1. Introduction to Chemical Bonding | 化学键简介

Atoms form bonds to achieve a more stable electronic arrangement, usually by obtaining a full outer shell of electrons. The three main types of strong chemical bond are ionic, covalent, and metallic. The type of bond formed depends on the electronegativity difference and the nature of the elements involved.

原子形成键是为了达到更稳定的电子排布，通常是通过获得满外层电子。三大强化学键为离子键、共价键和金属键。形成的键型取决于电负性差异和涉及元素的性质。

---

2. The Octet Rule and Electron Configurations | 八隅体规则与电子构型

The octet rule states that atoms tend to gain, lose, or share electrons so as to have eight electrons in their valence shell, resembling the electron configuration of a noble gas. For example, sodium (Na) has one outer electron and can lose it to become Na⁺ with the configuration of neon. Chlorine (Cl) has seven outer electrons and gains one to become Cl⁻, achieving argon’s configuration.

八隅体规则指出，原子倾向获得、失去或共享电子以使其价电子层具有八个电子，与稀有气体的电子构型相似。例如，钠（Na）有一个外层电子，可以失去成为 Na⁺，具有氖的构型；氯（Cl）有七个外层电子，获得一个成为 Cl⁻，达到氩的构型。

However, there are exceptions: hydrogen only needs two electrons (duet rule), and elements in period 3 or beyond can expand their octet due to available d-orbitals, such as in PCl₅ or SF₆.

但也有例外：氢只需两个电子（二隅体规则），第三周期及以上的元素由于有可用的 d 轨道可扩展八隅体，例如 PCl₅ 或 SF₆。

---

3. Ionic Bonding: Formation and Properties | 离子键的形成与性质

Ionic bonding occurs between metals and non-metals. Metal atoms transfer electrons to non-metal atoms, forming positive cations and negative anions. These oppositely charged ions are held together by strong electrostatic forces in a giant ionic lattice. For instance, sodium chloride forms when each Na atom donates one electron to a Cl atom, generating Na⁺ and Cl⁻ ions.

离子键形成于金属与非金属之间。金属原子将电子转移给非金属原子，形成正离子和负离子。这些带相反电荷的离子在巨型离子晶格中通过强静电引力结合在一起。例如，氯化钠由每个 Na 原子将一个电子给予 Cl 原子，生成 Na⁺ 和 Cl⁻ 离子而形成。

Ionic compounds are crystalline solids at room temperature with high melting and boiling points because a large amount of energy is needed to overcome the strong ionic bonds. They are brittle — when a force is applied, like charges align and cause the lattice to shatter. They conduct electricity only when molten or dissolved in water, as the ions become free to move.

离子化合物在室温下是晶体固体，熔点和沸点很高，因为需要大量能量来克服强大的离子键。它们脆性——当施加外力时，同性电荷排列导致晶格破碎。它们仅在熔融或溶于水时导电，因为离子可以自由移动。

---

4. Lattice Energy and Factors Affecting Ionic Bond Strength | 晶格能与离子键强度影响因素

Lattice energy is the energy released when one mole of an ionic crystal is formed from its gaseous ions. The more exothermic the lattice energy, the stronger the ionic bonding. Lattice energy becomes more negative when the ionic charges increase and when the ionic radii are smaller, because the ions can pack more closely and the electrostatic attraction intensifies. For example, MgO has a much higher melting point than NaCl due to the greater charge (Mg²⁺ and O²⁻ versus Na⁺ and Cl⁻) and smaller sizes.

晶格能是指由气态离子形成一摩尔离子晶体时所释放的能量。晶格能越负，离子键越强。离子电荷增加和离子半径减小时，晶格能更负，因为离子可以堆积得更紧密，静电吸引力增强。例如，MgO 的熔点远高于 NaCl，因为电荷更高（Mg²⁺ 和 O²⁻ 对比 Na⁺ 和 Cl⁻）且半径更小。

---

5. Covalent Bonding: Sharing Electrons | 共价键：电子共享

Covalent bonding typically occurs between non-metal atoms. Atoms share pairs of electrons so that each atom attains a stable outer shell. A single covalent bond shares one pair of electrons (e.g., H—H in H₂), a double bond shares two pairs (e.g., O=O in O₂), and a triple bond shares three pairs (e.g., N≡N in N₂). The shared electrons are attracted to the nuclei of both atoms, creating a strong directional bond.

共价键通常形成于非金属原子之间。原子通过共享电子对使每个原子获得稳定的外层。单键共享一对电子（如 H₂ 中 H—H），双键共享两对（如 O₂ 中 O=O），三键共享三对（如 N₂ 中 N≡N）。共享电子同时被两个原子核吸引，形成有方向性的强键。

Covalent substances can exist as simple molecular structures (like H₂O, CO₂) or as giant covalent lattices (like diamond and silicon dioxide). Simple molecules have strong covalent bonds within the molecules but weak intermolecular forces between them, leading to low melting and boiling points.

共价物质可以以简单分子结构（如 H₂O、CO₂）或巨型共价晶格（如金刚石和二氧化硅）存在。简单分子内部有强共价键，但分子间的分子间作用力弱，导致熔点和沸点低。

---

6. Dative Covalent (Coordinate) Bonds | 配位共价键

A dative covalent bond, also called a coordinate bond, is formed when both electrons in the shared pair come from the same atom. Once formed, it is indistinguishable from a normal covalent bond. A classic example is the ammonium ion NH₄⁺: ammonia (NH₃) uses its lone pair on nitrogen to bond with a hydrogen ion H⁺, which has an empty orbital. The resulting ion has four equivalent N—H bonds.

配位共价键（又称配位键）是指共享电子对的两个电子均来自同一个原子。一旦形成，它与普通共价键无法区分。一个典型例子是铵根离子 NH₄⁺：氨（NH₃）利用氮上的孤对电子与具有空轨道的 H⁺ 成键，所得离子含有四个等价的 N—H 键。

Other examples include the reaction between AlCl₃ and Cl⁻ to form AlCl₄⁻, and the bonding in carbon monoxide, CO, where one of the three bonds is dative.

其他例子包括 AlCl₃ 与 Cl⁻ 反应生成 AlCl₄⁻，以及一氧化碳 CO 中三键之一为配位键。

---

7. Shapes of Molecules and VSEPR Theory | 分子形状与 VSEPR 理论

The Valence Shell Electron Pair Repulsion (VSEPR) theory predicts molecular shapes by assuming that electron pairs around a central atom repel each other and arrange themselves as far apart as possible. Electron pairs can be bonding pairs or lone pairs; lone pairs exert greater repulsion, which reduces bond angles.

价层电子对互斥（VSEPR）理论预测分子形状，假设中心原子周围的电子对互相排斥并尽可能远离。电子对可分为键对和孤对；孤对的排斥力更强，因此会使键角减小。

For example: CO₂ has two bonding pairs and no lone pairs, giving a linear shape with 180°. BF₃ has three bonding pairs, giving trigonal planar with 120°. CH₄ has four bonding pairs, giving tetrahedral with 109.5°. NH₃ has three bonding pairs and one lone pair, producing a trigonal pyramidal shape with bond angles of about 107°. H₂O has two bonding pairs and two lone pairs, giving a bent shape with approximately 104.5°.

例如：CO₂ 有两对键对无孤对，为直线形，键角 180°；BF₃ 有三对键对，为平面三角形，键角 120°；CH₄ 有四对键对，为正四面体形，键角 109.5°；NH₃ 有三对键对和一个孤对，为三角锥形，键角约 107°；H₂O 有两对键对和两个孤对，为 V 形，键角约 104.5°。

Understanding VSEPR is crucial for predicting molecular polarity and reactivity. Always count the number of bonding and non-bonding electron pairs on the central atom to deduce the shape.

理解 VSEPR 对预测分子极性和反应性至关重要。始终先计算中心原子上键对和孤对的数量来推断形状。

---

8. Electronegativity and Bond Polarity | 电负性与键的极性

Electronegativity is the ability of an atom to attract the bonding pair of electrons in a covalent bond. When atoms with different electronegativities form a bond, the electron pair is unequally shared, creating a polar covalent bond. The more electronegative atom gains a partial negative charge (δ−), while the other atom gains a partial positive charge (δ+).

电负性是原子在共价键中吸引键合电子对的能力。当电负性不同的原子形成键时，电子对共享不均匀，产生极性共价键。电负性较大的原子带部分负电荷（δ−），另一个原子带部分正电荷（δ+）。

If the electronegativity difference is very large (usually >1.7 on the Pauling scale), the bond is considered ionic rather than covalent. Molecules with polar bonds may be overall non-polar if the dipoles cancel due to symmetry, as in CCl₄. In contrast, H₂O is polar because its bent shape gives a net dipole moment. Polarity influences physical properties such as solubility and boiling points.

若电负性差异非常大（鲍林标度通常 >1.7），则该键被认为是离子键而非共价键。含有极性键的分子，若因对称性使偶极抵消，则整体为非极性分子，如 CCl₄。而 H₂O 因 V 形结构具有净偶极矩，为极性分子。极性影响溶解度和沸点等物理性质。

---

9. Intermolecular Forces: London, Dipole-Dipole, Hydrogen Bonding | 分子间作用力：伦敦力、偶极-偶极力与氢键

Intermolecular forces are weak attractions between molecules, distinct from the strong intramolecular covalent bonds. They determine bulk properties like melting and boiling points of simple molecular substances. The three main types are London dispersion forces, permanent dipole-dipole interactions, and hydrogen bonding.

分子间作用力是分子之间的弱吸引力，有别于分子内强共价键。它们决定简单分子物质的熔沸点等宏观性质。主要有三种类型：伦敦色散力、永久偶极-偶极相互作用和氢键。

London dispersion forces exist between all molecules due to instantaneous fluctuations in electron distribution, creating temporary dipoles. Larger molecules with more electrons exhibit stronger London forces, resulting in higher boiling points. Permanent dipole-dipole forces occur between polar molecules, where the δ+ end of one molecule attracts the δ− end of another. These are stronger than London forces but still relatively weak.

伦敦色散力存在于所有分子之间，源于电子分布瞬间涨落产生瞬时偶极。分子越大、电子越多，伦敦力越强，沸点越高。永久偶极-偶极力存在于极性分子之间，一个分子的 δ+ 端与另一个分子的 δ− 端相吸。其作用强于伦敦力，但仍相对较弱。

Hydrogen bonding is a special strong dipole-dipole interaction that occurs when hydrogen is covalently bonded to highly electronegative atoms such as nitrogen, oxygen, or fluorine. The lone pair on N/O/F interacts with the electron-deficient hydrogen, creating a bond roughly one-tenth the strength of a covalent bond. Hydrogen bonding explains the unusually high boiling points of H₂O, HF, and NH₃ and the structure of ice and DNA.

氢键是一种特殊的强偶极-偶极相互作用，发生在氢与氮、氧或氟等高电负性原子共价结合时。N/O/F 上的孤对电子与缺电子的氢作用，形成强度约为共价键十分之一的键。氢键解释了 H₂O、HF 和 NH₃ 异常高的沸点以及冰和 DNA 的结构。

---

10. Metallic Bonding: Sea of Electrons | 金属键：电子海模型

Metallic bonding occurs between metal atoms. Metal atoms lose their outer electrons to form a lattice of positive metal ions immersed in a ‘sea’ of delocalised electrons. These delocalised electrons are free to move throughout the entire structure and are not associated with any particular ion, which explains the characteristic properties of metals.

金属键发生在金属原子之间。金属原子失去外层电子，形成沉浸在“海洋”般的离域电子中的正金属离子晶格。这些离域电子可在整个结构中自由移动，不属于任何特定离子，这解释了金属的特征性质。

Metals are good conductors of electricity and heat because delocalised electrons can carry charge and kinetic energy quickly. They are malleable and ductile because layers of ions can slide over each other without breaking the metallic bonding — the delocalised electrons act as a glue that readjusts. The strength of metallic bonding increases with the number of delocalised electrons per atom and with the charge density of the ion, giving trends in melting points such as the high melting point of transition metals compared to group 1 metals.

金属是电和热的良导体，因为离域电子能迅速传递电荷和动能。金属具有延展性，因为离子层可在不断裂金属键的情况下相互滑动——离域电子起到可重新调节的“胶水”作用。金属键强度随每个原子离域电子数及离子电荷密度增加而增大，这导致了熔点的变化趋势，例如过渡金属的熔点远高于第 1 族金属。

---

11. Comparison of Bonding Types and Physical Properties | 键型与物理性质比较

Ionic solids have high melting points and are brittle, conducting electricity only when molten or aqueous. Giant covalent networks like diamond have extremely high melting points and are generally non-conductors (except graphite due to delocalised electrons). Simple molecular substances have low melting points and are non-conductors because there are no free ions or electrons. Metals have a range of melting points, are malleable, and are excellent conductors in all states.

离子固体的熔点高、性能，仅在熔融或水溶液中导电。巨型共价网络如金刚石熔点极高，一般不导电（石墨因有离域电子除外）。简单分子物质熔点低，是不良导体，因为没有自由离子或电子。金属熔点范围广，具延展性，且在任意状态下均为优良导体。

Understanding these contrasts enables you to deduce an unknown substance’s bonding type from its properties, a common exam task. For instance, a solid that dissolves in water and the solution conducts electricity is probably ionic. A solid that conducts as a solid is likely metallic.

理解这些差异能让你从性质推断未知物质的键型，这是常见的考试任务。例如，一种固体溶于水后溶液导电，则很可能是离子型；固态即导电的固体则很可能是金属。

---

12. Summary and Revision Tips for Exams | 总结与备考技巧

Focus on the relationships between bonding, structure, and properties. Memorise the VSEPR shapes and bond angles; practise drawing dot-and-cross diagrams for ionic and covalent compounds. Be able to explain trends in melting points using lattice energy or intermolecular force arguments. Write balanced equations for the formation of dative bonds, such as NH₃ + H⁺ → NH₄⁺. Use electronegativity to deduce bond type and explain molecular polarity.

重点掌握键合、结构与性质之间的关系。熟记 VSEPR 形状和键角；练习绘制离子和共价化合物的电子式点叉图。能用晶格能或分子间作用力解释熔点变化趋势。书写配位键形成的方程式，如 NH₃ + H⁺ → NH₄⁺。运用电负性推断键型并解释分子极性。

When answering exam questions, always link your explanation to the particles (ions, molecules, or atoms) and the forces between them. Practise past paper questions on bonding to become confident in identifying the key concepts quickly.

回答考试问题时，务必将解释与微粒（离子、分子或原子）及其间的作用力联系起来。多练习往年真题中的键合题目，以便快速准确识别关键概念。

Published by TutorHao | Combined Science Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）